Hitherto, there has been known to use a tensioner for applying tension to a timing chain for driving camshafts and others of an engine by exerting ejection force on a plunger, slidably and fittingly inserted into a housing and forming an oil chamber with the housing, by a spring and external hydraulic pressure.
As such prior art tensioner, there is adopted a ratchet-type tensioner 500 as shown in FIG. 18 for example. The ratchet-type tensioner 500 comprises a housing 512, a plunger 514 slidably accommodated in the housing 512, a piston 526 slidably and fittingly inserted into the housing 512 in a direction traverse a sliding direction of the plunger 514 and forming a oil sub-chamber 520 between the housing 512 and the piston 526, an oil passage 544 for exerting external hydraulic pressure to the oil sub-chamber 520, a second spring 534 for biasing the piston 526 toward the oil sub-chamber 520, an air chamber 528 provided on the side opposite from the oil sub-chamber 520, sectioned and formed by the housing 512 and the piston 526 and including the second spring 534, a cap 530 fitted into the air chamber 528, an air communicating hole 532 provided in communication with the air chamber 528 and closed by the piston 526 when the external hydraulic pressure acts on the oil sub-chamber 520 and the piston 526 moves against a biasing force of the second spring 534, rack teeth 538 engraved on a part of the plunger 514 surrounded by the housing 512, and a plurality of engaging teeth 536 provided at an edge of a rod 524 fixed to the piston 526 and capable of engaging with the rack 538, wherein plunger-setback blocking tooth surfaces of the engaging teeth 536 and the rack 538 are formed at right angles with respect to a direction in which the plunger 514 advances/sets back (see Patent Document 1 for example).